Chore: change Next.js build output folder

[nickytonline]

Following #4094 and #4096, we might want to consider a different build output folder for Next.js, like build instead of the default, .next and revert the changes in #4096.

I didn't do this in #4096 in case it caused other issues in the CI/CD pipeline. I think it should be fine, but just need to confirm on the Netlify deploy side of things.